This invention relates generally to a caliper and, more particularly, to a skin-fold caliper of simple construction in which the jaws thereof are biased together by a spring.
Although this invention has particular application in the field of bariatric medicine for the study and treatment of obesity, it can be appreciated that its use need not be so limited. The caliper disclosed herein can be utilized by physiologists and bariatricians to determine what percentage of the human body is fat and what percentage is actually vital body tissue and muscle. An accepted technique for determining the percentage of fat has been to measure the hydrostatic weight (underwater weight) of the person under study. However, weighing a person underwater can be extremely inconvenient.
It is widely known that the human body stores 50 percent or more of its fat under the surface of the skin and that as the percentage of total fat increases, the thickness of the subsurface layers of fat increases in substantially equal proportion. Thus, if assessments are made of the subsurface layers by measuring the thickness of skin folds, it is possible to estimate the total amount of fat accumulated by the body.
Typically, skin-fold measurements are taken by pinching the skin and the flexible subtissue below the skin at certain selected positions on the body and the limbs. For women, these measurements can be taken at the back of the arm (tricep) and at the hip (iliac crest). For men, additional measurements may be taken at the front of the arm (bicep) and at the upper back (subscapular). The sum of these measurements is then compared with a chart to determine the relative percentage of fat.
It should be evident that because of the resiliency of the skin and the subsurface layer of fat, the measurements obtained will necessarily be a function of the pressure applied by the caliper. The generally accepted standard pressure for taking skin-fold measurements of the human body is 10 grams per square millimeter.
Presently, constant pressure calipers designed to measure skin folds are available, one such device is illustrated in Lange U.S. Pat. No. 3,008,239. However, this device is relatively complex and, therefore, prohibitively expensive. The Lange calipers utilize a spring to apply pressure between the jaws thereof. However, since a spring generates a force which varies proportionately with the distance to which it is extended, Lange employs a gear and lever system to obtain constant pressure over the range of distances through which the jaws may be extended.
By constructing a caliper having jaws which move linearly away from each other and by employing a coiled tension spring having desirable design parameters, a caliper can be simply and inexpensively manufactured. It has been found that slight pressure changes do not significantly affect the accuracy of skin-fold measurements particularly where skin-fold thickness is, in practice, only an approximation of actual fat accumulation. Therefore, a linear spring-operated caliper capable of taking relatively accurate measurements is feasible provided that the spring is appropriately selected so that changes in spring extension effect only insignificant changes in applied spring force.